1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus and, more particularly, to a fixing device that includes a cleaning unit that presses an elongated cleaning web running from a supply core to a take-up core against a fixing roller of a fixing device that includes the fixing roller provided with a heat source and a pressure-applying roller that is in pressure contact with the fixing roller, thereby cleaning the fixing roller.
2. Description of the Related Art
An image forming apparatus such as a copier, a printer, or a facsimile typically employs a fixing device of a heating-roller type that fixes a not-yet-fixed toner image on a recording medium, such as transfer paper, with heat and pressure by bringing a fixing roller (i.e., a heating roller) and a pressure-applying roller into pressure contact with each other and causing the recording medium to pass through between the rollers.
Known examples of this conventional type of fixing device include a fixing device disclosed in Japanese Patent Application Laid-open No. 2003-255745. This fixing device includes a web-type cleaning unit that cleans a surface of a fixing roller by bringing the fixing roller into sliding contact with a cleaning web that is being taken up from a supply core, on which the cleaning web is wound, onto a take-up roller to prevent toner from being peeled off from transfer paper and adhering to the fixing roller (toner offset).
The web-type cleaning unit uses heat-resistant fiber such as nonwoven textile impregnated with a releasing agent such as silicone oil as the cleaning web, thereby not only wiping off toner adhering to the surface of the fixing toner with the cleaning web but also supplying the releasing agent from the cleaning web to the surface of the fixing roller. The web-type cleaning has cleaning performance superior to other type of cleaning such as roller-type cleaning that brings a cleaning member, a roller, into contact with a surface of a fixing roller or felt-type cleaning that brings a cleaning member made of felt into sliding contact with a fixing roller.
The cleaning unit of the conventional fixing device is configured to maintain cleaning performance by feeding only a minuscule length (1 mm, for example) of the cleaning web each time a predetermined number of recording sheets are subjected to fixing operation so that a new portion, which is impregnated with silicone oil, of the cleaning web is regularly brought into sliding contact the fixing roller.
However, the web-type cleaning unit of the fixing device disclosed in Japanese Patent Application Laid-open No. 2003-255745 has a disadvantage. That is, because oil seeps from an outer-radius portion of the rolled-up cleaning web and moves toward the take-up core which is a core of the cleaning web, an amount of oil absorbed and retained in the cleaning web wound on the take-up core in a roll form is large in an inner-radius portion but small in the outer-radius portion.
For example, as illustrated in FIGS. 5A and 5B, when a cleaning web has an overall length of 32 m, an amount of oil absorbed and retained in a portion of the cleaning web, which corresponds to an outer-radius portion of the cleaning web wound on a take-up core, within 0.5 m from a leading end of the cleaning web is 10% to 20% smaller than a lower threshold of a target range, while an amount of oil absorbed and retained in a portion of the cleaning web, which corresponds to an inner-radius portion of the cleaning web wound on the take-up core, within 0.5 m from a trailing end of the cleaning web is 10% to 20% larger than an upper threshold of the target range.
There is a problem in the portion of the cleaning web within 0.5 m from the leading end where the amount of oil is small. That is, because oil on the surface of the fixing roller becomes insufficient and cleaning performance is delivered insufficiently, toner offset onto the surface of the fixing roller occurs, which undesirably results in contamination of an image.
Furthermore, when toner offset onto the surface of the fixing roller occurs, the moved toner can be accumulated on a thermistor that contacts the fixing roller and undesirably damage a surface layer of the fixing roller.
There is also a problem in the portion of the cleaning web within 0.5 m from the trailing end where the amount of oil is large. That is, because oil on the surface of the fixing roller becomes excessive, toner escaping occurs, which undesirably results in contamination of an image.
Put another way, there is a problem that because oil absorbed and retained in the cleaning web is deficient in the portion near the leading end and excessive in the portion near the trailing end, the surface of the fixing roller becomes nonuniform in anti-adhesive properties.
Therefore, there is a need for a fixing device and an image forming apparatus capable of equalizing anti-adhesion properties across a surface of a fixing roller whichever portion of a cleaning web is fed to clean the surface.